Was it?
I mean, that's how it's taught in schools, but how well does that reflect the actual history? Certainly it doesn't represent the UK's history with Ireland, which really isn't taught in schools. That's how people like Melanie "wrong about everything all of the time" Phillips can write national newspaper articles in which Ireland has a "tenuous claim to nationhood".
This is kind of the problem, we have all this national mythos that runs up to about 1966 and then stops. And it doesn't bear scrutiny in the modern age. The last attempt at building a national narrative that worked for everyone was the Blair era "cool Britannia", and half a million dead Iraqis took the shine off that a bit.
Not to mention the inward focus of Unionism. Everyone looks towards London, but London doesn't look outwards. Occasionally it dispatches a foreign correspondent to Manchester to report on conditions. The average English person thinks of Scotland little and NI not at all.
How it's taught in schools in UK/England perhaps. In Ireland the history class is all about how England is a horrible oppressor of Ireland. ;)
"Ethnic countries" - if the states primarily contains one nation of people - ethnically (ie mainly culturally), is commonly referred to as a national state.
National states became a popular concept in the 19th century, possibly because the kings, emperors and other rulers were starting to loose power and had to build their states around some concept. Before that, the kings and the rulers did not give the nationality of their subjects much of a thought I suppose.
Even worse for me, I'm an internationalist and I felt very proud to be part of the EU for many of the same reasons I felt proud to be part of the UK: civic nationalism. The UK was the worlds most successful civic nation built on mutualism between a number of ethnic countries. To break up on ethnic borders and return to tribalism is heartbreaking.